1. Field of the Invention
The present invention relates to a method of construction of a sealing ring and to a product obtained in accordance with the method.
More particularly, the invention relates to a sealing ring which is suitable for use in fluidic circuits, especially those of thermal power plants.
2. Description of the Prior Art
In thermal power plants, the dimensions of the various equipment units such as control valves and the steam pressures within these units have now reached values such that, in the case of large diameters and high pressures, equipment manufacturers have replaced the conventional assembly of bolted external coupling flanges by a built-in system.
For example in the case of a valve equipped with a plug and an operating stem which passes through the plug, it is necessary to provide a sealing system within the closed space delimited by an inclined portion of one plug face, the internal wall of the valve body and a single-piece ring which is held by a detachable upper ring fitted within an annular groove of the valve body. As a rule, the inclined portion of the plug whose bottom and top faces are subjected to the pressure of circulating fluid and to the external pressure respectively is inclined to the vertical axis of the plug at an angle of 25.degree..
The sealing system which was still in use a short time ago consisted of a metal ring of suitable shape which was placed within the above-mentioned closed space and had a lower portion or lip which was capable of undergoing deformation at the time of displacement of the plug produced by the pressure rise within the body of the device. The deformation of the lip was sufficient to ensure leak-tightness. However, when leakage occurred prior to deformation of the lip, erosion of the valve body and/or of the plug was liable to occur, thus inevitably giving rise to deficient leak-tightness.
Moreover, at the time of removal of the plug, the forces which have been generated by the hydrostatic pressure exerted on the internal surface of the plug and which have been entirely transmitted to the lip of the ring have deformed the lip to a degree such that the force exerted on the plug towards the interior of the device during the operations involved in disassembly in order to release the different rings is liable to cause eccentric displacement of the plug and jamming against the body of the device. This would entail the need to heat or to saw the body in order to cause it to expand, that is to say to cause partial destruction of a device having walls which were intended to afford resistance to high pressures.
In order to overcome these disadvantages, it has recently been proposed to replace the metal rings by recompressed rings of expanded graphite.
However, sealing rings of this type having a base of graphite of variable thickness have proved suitable for use on the one hand only in small volumes or closed spaces to be filled and on the other hand in devices in which the fluid is substantially stable in pressure and in temperature. The reason for this is that, in the case of large spaces to be filled, it is not possible to gain complete mastery over the technique of manufacture of an expanded graphite ring of variable thickness which performs the double function of resistance to pressure and leak-tightness.
In fact, during compression of the graphite, the dimensions vary considerably in the direction of compression and the paths of the graphite particles remain very close to straight lines parallel to the cylindrical walls. In consequence, the lowest degrees of densification will correspond to the largest dimensions in the direction of compression.
The aim of the present invention is to produce a sealing ring which has a base of expanded graphite and which has good characteristics of pressure resistance and leak-tightness.